Complex organisms use chemical signals to integrate and coordinate the function of specialized tissues. In many cases, information is transferred in the form of chemicals, e.g., polypeptide hormones or growth factors, which interact with target cells through specific, surface receptors and profoundly affect cell structure and function. This project is concerned with the question of how such information flows to intracellular metabolic control points, i.e., defining postreceptor signalling mechanisms. Recent-research has focussed on the role of intracellular receptors in the overall mechanism of insulin action. For these studies, we have developed a powerful experimental system consisting of a giant, insulin-sensitive cell (amphibian oocyte), paraffin oil-based cell microinjection and fractionation procedures and single cell microanalysis. Using this system, we have obtained the first evidence that intracellular insulin can directly affect cell metabolism. That is, microinjected, intracellular insulin stimulates both transcription and translation by acting at nuclear and cytoplasmic sites. Such actions are independent of the surface receptor, since they also occur in isolated nuclei and cytoplasm samples. Insulin action at internal sites also occurs when cells are exposed to extracellular hormone, since the insulin-stimulated component of protein synthesis is partially blocked when cells are first exposed to external hormone and then microinjected with antibody. Inhibition by antibody is only found in cells which have accumulated undegraded insulin intracellularly. Future plans include 1) defining the mechanisms by which insulin gains access to the internal receptors and through them affects metabolism, 2) determining the extent to which intracellular receptors are involved in the action polypeptide hormones and growth factors in somatic cells, and 3) assessing the role of disrupted intracellular signalling in pathological states, e.g., insulin resistance.